1. Field of the Invention
The present invention relates to composite materials, and particularly, but not exclusively, to fibre reinforced composite materials.
2. Description of Related Art
Composite materials are increasingly used in structural applications in many fields owing to their attractive mechanical properties and low weight in comparison to metals. Composites are known in the field to consist of layering of materials to provide a structurally advantageous laminate type material. However, whilst electrical conductivity is one of the most obvious attributes of metals, composite materials based on fibre reinforcements (such as adhesive films, surfacing films, and pre-impregnated (prepreg) materials), generally have much lower electrical conductivity.
Conventional composite materials usually consist of a reinforcement phase, generally comprising continuous or discontinuous fibres, and a matrix phase, generally a thermoset or thermoplastic polymer. Most early first generation matrix polymers for the manufacture of composites were, by nature, brittle and it has therefore been necessary to develop more toughened versions. The composites materials used as primary structures in aerospace applications tend to be so-called second or third generation toughened materials.
There is a particular need for composite materials which exhibit electrical conductivity for several applications. These applications include use for protection against lightning strikes, electrostatic dissipation (ESD), and electromagnetic interference (EMI). Prior composite materials, such as those based upon carbon fibres, are known to have some degree of electrical conductivity which is usually associated with the graphitic nature of the carbon filaments. However, the level of electrical conductivity provided is insufficient for protecting the composite material from the damaging effects of, for example, a lightning strike.
Second generation toughened composites represent an improvement over earlier first generation materials due to incorporation of toughening phases within the matrix material. Various methods for increasing electrical conductivity in these composites have been used. These methods typically include incorporation of metals into the assembly via expanded foils, metal meshes, or interwoven wires. Typical metals which are used for this purpose include aluminium, bronze and copper. These composite materials can provide better electrical conductivity. However, they are generally heavy and have significantly degraded mechanical and aesthetic properties. These composites are usually found at the first one or two plies of the material, and therefore a poor overall surface finish often results.
In the event of a lightning strike on second generation composites, damage is normally restricted to the surface protective layer. The energy of the lightning strike is typically sufficient to vaporise some of the metal and to burn a small hole in the mesh. Damage to the underlying composite may be minimal, being restricted to the top one or two plies. Nevertheless, after such a strike it would be necessary to cut out the damaged area and make good with fresh metal protection and, if required, fresh composite.
As already mentioned, materials with carbon fibres do possess some electrical conductivity. However, the conductivity pathway is only in the direction of the fibres, with limited ability for dissipation of electrical current in directions orthogonal to the plane of the fibre reinforcement (z direction). Carbon reinforced materials often comprise an interleaf structure which results in inherently low conductivity in the z direction due to the electrical insulation properties of the interleaf. The result of such an arrangement can lead to disastrous effects when damaged by lightning as the electrical discharge can enter the interleaf, volatilise the resin therein, and cause mass delamination and penetration through the composite material.
So-called third generation toughened composite materials are based on interleaf technology where resinous layers are alternated with fibre reinforced plies, and provide protection against impacts. However, these resin layers act as an electrical insulator and therefore electrical conductivity in the z direction of the material is poor (i.e. orthogonal to the direction of the fibres). Lightning strikes on the composite material can result in catastrophic failure of the component, with a hole being punched through a multiple ply laminate.